Call duration

Call duration is the period from when the call is answered, possibly automatically (by an answering machine, for instance), until the call is terminated by either the interviewer or the respondent hanging up.

	Base	Project A	Project B
Base	269724 100%	200962 100%	68762 100%
Minimum: Duration	*	*	*
Maximum: Duration	2416	2416	1287
Mean: Duration	33.4	35.0	28.8
Standard Deviation: Duration	80.43	87.00	56.77
Lower quartile	5.2	5.8	4.2
Median	20.6	22.7	12.5
Upper quartile	38.1	39.1	34.1

These results show that although most calls are surprisingly short there is a small number of comparatively long calls. This is visible in the mean, standard deviation, and median values. The mean call length is around 29 to 35 seconds which is not much longer than the no answer time-out. The deviation from the mean is very large. The 75th percentile is 34 and 39 seconds for Project A and Project B respectively which is only slightly longer than the mean call length, and which indicates that the mean call length is boosted by a small number of long duration calls. Calls for Project B are generally shorter than those for Project A.

Other observations are:

▪Around 30% to 40% of calls are less than 8 seconds. Some of these will be due to fax/modems as the call is answered and then terminated automatically when a fax/modem is detected. Other short calls are likely to be due to answering machines or other automated announcements and calls where the respondent hangs up immediately.

▪Around 55% to 65% of calls are eight to 90 seconds in length, presumably due to refusals and interviews that start but are terminated early due to quota requirements.

The two charts below show the same data as before but over a period of around five minutes in order to show what makes up the "over range" bar on the preceding charts. It is obvious from these charts that very few calls are longer than 90 seconds.